Chlamydiae Has Contributed at Least 55 Genes to Plantae with Predominantly Plastid Functions

Contributed equally to this work with:
Ahmed Moustafa,
Adrian Reyes-Prieto

Affiliations
Interdisciplinary Program in Genetics, University of Iowa, Iowa City, Iowa, United States of America,
Department of Biological Sciences and Roy J. Carver Center for Comparative Genomics, University of Iowa, Iowa City, Iowa, United States of America

Affiliations
Interdisciplinary Program in Genetics, University of Iowa, Iowa City, Iowa, United States of America,
Department of Biological Sciences and Roy J. Carver Center for Comparative Genomics, University of Iowa, Iowa City, Iowa, United States of America

Why present-day Plantae have no chlamydial symbionts?

It seems that all known chlamydiae are obligate intracellular symbionts - they can only reproduce inside eukaryotic cells, and remain metabolically inactive outside of their hosts (a virus-like lifestyle). Chlamydiae can infect different kinds of animals (mammals, birds, fishes, arthropods, crustaceans) and unicellular eukaryotes (such as environmental amoebae). Remarkably, chlamydiae have never been found in Plantae (red and green algae, plants, glaucophytes), so far.

On the other hand, this article and others show that an important number of Plantae genes are derived from ancient chlamydiae, suggesting a long-term symbiosis between ancestors of chlamydiae and Plantae. The bacterial endosymbiont was later lost, leaving some of its genes behind.

So, the question is: why present-day Plantae have no chlamydial symbionts (parasitic, mutualistic, commensal)? Why can't they be infected, once more, by chlamydiae? I may think of some possible answers:

(1) Actually, there are chlamyidial symbionts in Plantae. We just haven't found them. (Have we looked for them?)

(2) Plantae are not special: other eukaryotic lineages appear to lack chlamydial symbionts (just a hypothesis, I have no idea). In other words, chlamydiae are able to infect cells from only specific eukaryotic lineages. (How wide is chlamydial host-range? Has anybody tried to infect Plantae cells with chlamydiae?)

(3) Plantae are special: after being infected by chlamydia-like bacteria, the Plantae lineage became resistant to over-infection by other chlamydiae. (If this is the case, it would be nice to know the molecular mechanisms responsible for the resistance. Can we make a Plantae cell susceptible to infection by knocking-out specific genes? Are these genes derived from chlamyidiae? Or, the other way around, can we make a Plantae cell susceptible to infection by adding specific genes from other eukaryotes? This knowledge could be useful to design new anti-chlamydial therapies.)

RE: Why present-day Plantae have no chlamydial symbionts?

I think you raise an interesting question. While I am not able to pinpoint a specific host, some of my research suggests that Chlamydiae infect phytoplankton. I set up an algal exchange experiment where I incubated bacteria (lake water passed through a 1 micron capsule filter) from 2 different humic lakes with native phytoplankton assemblages from one of the lakes or no phytoplankton as a control. To collect phytoplankton assemblages, lake water was passed through a 100 micron mesh to remove larger zooplankton and a 20 micron filter to catch the phytoplankton and rinse away free-living bacteria and smaller protists. One caveat to this method is that there were a small number of rotifers (Keratella) included with phytoplankton. I used tag-pyrosequencing of the 16S rRNA gene to identify bacteria that responded to the phytoplankton treatments. Before the incubation, hardly any Chlamydiae sequences were detected, but after incubation, one OTU that was identified to the order Chlamydiales made up 2-13% of the bacterial sequences from mesocoms with phytoplankton from one of the two lakes while remaining below detection in mesocosms with phytoplankton from the other lake or no phytoplankton. Part of the experiment was to see how bacterial communities responded to phytoplankton under different temperature and light conditions and the Chlamydiales OTU was not detected in mesocosms incubated at the colder temperature (around 10°C) and at the warmer temperature (around 25°C) was particularly abundant in mesocosms in the ambient light treatment (low light was ~0.5 meters below the surface of a humic lake and received about 25% of the surface illuminance).